This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Ward Leonard Speed Control Method”.
1. Ward Leonard method is ___________
a) Armature control method
b) Field control method
c) Combination of armature control method and field control method
d) Totally different from armature and field control method
View Answer
Explanation: Ward Leonard method is the combination of armature control method and field control method, which can also be called as voltage control method. This is the most efficient method of speed control over wide range.
2. Which of the following component is not used in Ward Leonard method?
a) AC motor
b) DC generator
c) DC motor
d) AC generator
View Answer
Explanation: Whole unit of Ward Leonard speed control unit consists of various units like DC generator, DC motor, AC motor, exciter circuit and various pots which are used for carrying out smooth operation.
3. In Ward Leonard speed control method for lowering the speed of the motor ______________
a) Reduce armature voltage
b) Increase armature voltage
c) Increase field current
d) Decrease field current
View Answer
Explanation: In Ward Leonard speed control method, speed can be reduced under base value by reducing armature voltage. By increasing field current speed can be reduced but this is not employed in Ward Leonard method.
4. Reducing the armature voltage will give us _______________
a) Variable torque speed control
b) Constant torque speed control
c) Variable and constant both can be achieved
d) Cannot comment on torque
View Answer
Explanation: As seen from speed torque characteristics, reducing armature voltage will reduce the speed of the motor below base value but torque will remain same. Thus, reducing armature voltage will give constant torque speed control.
5. In Ward Leonard speed control method for increasing the speed of the motor ______________
a) Reduce armature voltage
b) Increase armature voltage
c) Increase field current
d) Decrease field current
View Answer
Explanation: In Ward Leonard speed control method, speed can be increased above base value by weakening of the field, which can be done by lowering field current value. By increasing armature voltage speed can be increased but this is not employed in Ward Leonard method.
6. Reducing the field current will give us _______________
a) Constant torque and variable power speed control
b) Constant torque speed control
c) Variable power speed control
d) Constant power speed control
View Answer
Explanation: As seen from speed torque characteristics, reducing field current will increase the speed of the motor above base value but power will remain same. Thus, reducing armature voltage will give constant power speed control, with variable torque.
7. Speed-power characteristic for Ward Leonard speed control method _________________
a) Will start from origin
b) Will start from some positive value on power axis
c) Will start from some positive value on speed axis
d) Depends on other parameters
View Answer
Explanation: Speed power characteristic of DC motor is plotted when, Ward Leonard speed control method is employed. For speed equal to zero, which is less than base speed, we get constant torque but variable power operation. Thus, power will start increasing from origin.
8. Efficiency of Ward Leonard method is ____________
a) Higher than rheostatic control method but lower than shunted field control method
b) Lower than rheostatic control method
c) Higher than rheostatic control method and shunted field control method
d) Depends on load
View Answer
Explanation: Unlike all other methods, external resistance is not added in the circuit of control system. Thus, efficiency of Ward Leonard control method is always highest at various different speeds.
9. Ward Leonard method is an ideal choice for motor which undergoes frequent starting, stopping, speed reversal.
a) True
b) False
View Answer
Explanation: Absence of external resistance increases efficiency. Also, when the generator emf becomes less than the back emf of the motor, electrical power flows back from motor to generator, is converted to mechanical form and is returned to the mains via the driving ac motor. This aspect makes Ward Leonard method perfect for given application.
10. Starting gear used in Ward Leonard method___________
a) Is of small size
b) Is of large size
c) Size depends on application
d) Is absent
View Answer
Explanation: No special starting gear is required in Ward Leonard method of speed control. As the induced voltage by generator is gradually raised from zero, the motor starts up smoothly. Speed reversal is smoothly carried out.
11. To get the speed of DC motor below the normal speed without wastage of electrical energy we use __________________
a) Ward Leonard control
b) Rheostatic control
c) Any of the Ward Leonard or rheostatic method can be used
d) Not possible
View Answer
Explanation: Ward Leonard method of speed control is most efficient method of speed control in all aspects. We can get constant torque operation and constant power operations as well, with this method.
12. Speed control by Ward Leonard method, can give uniform speed variation _______________
a) In both directions
b) In one direction
c) Below normal speed only
d) Above normal speed only.
View Answer
Explanation: Speed control by Ward Leonard method, gives uniform speed variation in both the directions and above and below of normal speed as well. Speed reversal is carried out smoothly by this control method.
13. Ward Leonard control is basically a _____________
a) Voltage control method
b) Field diverter method
c) Field control method
d) Armature resistance control method
View Answer
Explanation: Ward Leonard speed control method is combination of rheostatic series control method and shunted armature control method with field control as well. Thus, it can be called as voltage control method also.
14. In Ward Leonard control of DC motor, the lower limit of speed is imposed by ____________
a) Residual magnetism of the generator
b) Core losses of motor
c) Mechanical losses of motor and generator together
d) Cannot be determined
View Answer
Explanation: We get the speed below the base value by reducing armature voltage, which is the simple method of reducing back emf which is proportional to the residual magnetism. Thus, lower limit of speed is imposed by residual magnetism of the generator.
15. The disadvantage of the Ward Leonard control method is ___________
a) High initial cost
b) High maintenance cost
c) Low efficiency at high loads
d) High cost, high maintenance and low efficiency
View Answer
Explanation: Ward Leonard speed control method requires large number of building blocks like generators, motors. Thus, installing cost and maintenance cost of the whole unit is very high. Apart from cost, it gives low efficiency at very high loads.
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